Mapping of the RNAs of defective interfering (DI) particles of vesicular stomatitis virus (VSV) by means of annealing experiments has shown that the Canadian HR DI particle RNA (50% of viral genome) corresponded to complementary sequences of all the 18-13S mRNA species. In contrast all other DI particles (generated by Glasgow ts mutants and various US wild types) available to us had RNA species complementary to a part or all of the viral 30S mRNA. Of special interest were the ts 11 and ts 22 generated DI particles whose RNA is comparable in size to the HR DI particle RNA. The two former particles contained RNA corresponding to the entire 30S viral cistron and unlike the HR particle were unable to interfere heterotypically with the distantly related New Jersey serotype of VSV. The latter property is therefore linked with the presence of some or all of the sequences complementary to the 18S-13S viral cistrons. In order to localize this property more narrowly, DI particles generated by the Canadian temperature sensitive mutants which derived from the HR isolate will be isolated, mapped and checked for heterotypic interference. Partial nucleotide sequence determinations by biochemical methods are also in progress in search of short regions (missed in annealing experiments) common to all DI particles and possibly responsible for the high specificity of autointerference.